What the A.400 alarm means

The main-circuit DC bus voltage exceeded the limit (about DC 420 V on 400 V class units); the SERVOPACK stops to protect the capacitors and power stage. In our Yaskawa service archive this alarm on DX100 / DX200 / YRC1000 systems shows up constantly, and the first on-site inspection almost always locates it.

Most common causes

  • Incoming AC supply voltage too high
  • Large regenerative energy during deceleration with a weak or missing braking resistor
  • Unstable plant power with voltage spikes
  • SERVOPACK detection circuit fault
Field note: The first cause is the most common in the field — check it first, then work through the rest.

Step-by-step field fix

  1. Measure incoming AC voltage and verify it is within the rated range
  2. Check the braking resistor and its wiring (an open resistor turns every decel into overvoltage)
  3. Look for voltage spikes from other heavy equipment on the same feeder
  4. Reduce deceleration rates and retry the motion
  5. Replace the SERVOPACK only after supply and regeneration paths are proven healthy
Safety first: For YASKAWA a-400, this alarm family involves the DC bus, which sits at several hundred volts and stays charged for minutes after power-off - touching terminals is a shock hazard. The self-serviceable action is Measure incoming AC voltage and verify it is within the rated range; anything past that falls under the DC bus and power stage and needs a certified engineer. On YASKAWA a-400: checking incoming voltage at the disconnect and looking at breakers and fuses is fine to do yourself; opening the cabinet to touch bus terminals or pulling power modules needs a qualified electrician.

How we diagnose drive alarms on site

YASKAWA a-400 belongs to the drive & power stage family. On YASKAWA a-400 the failing stage is Yaskawa Sigma-series servo drives (servo packs) assigned per axis in the cabinet on the DX100 / DX200 / YRC1000 / YRC1000micro - the fault sits on the energy chain: mains input, rectification, DC bus, braking resistor, inverter power devices and the drive board. We start reading YASKAWA a-400 at the pendant alarm list (A / E / 4xxx segments) together with the servo drive's digit display.

The chain involved in YASKAWA a-400 runs: three-phase mains -> rectifier -> DC bus (bus capacitors) -> brake chopper / braking resistor -> inverter (IGBT) -> motor.

When YASKAWA a-400 fires, the site usually reports Incoming AC supply voltage too high first, but the real cause is often cooling: More than half of power-stage alarms are not a dead device but a device cooked by failed cooling. A stopped cabinet fan, a packed filter mat or a dusted air duct keeps IGBT junction temperature high: intermittent over-temperature first, hard fault days later. On these alarms we open the power module and look for cracked thermal paste and felted dust in the duct, then measure phase-to-phase output and insulation to frame. Swapping the module without cleaning the duct just buys the same failure again.

Field notes on this page cover YASKAWA a-400 (Overvoltage — Main Circuit DC Voltage Too High); written by the Henghuan repair team, updated 2026-10-01.

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Related Drive & Power Stage alarms

A.100 — Overcurrent or Heat Sink Overheated (SERVOPACK) A.320 — Regenerative Overload A.410 — Undervoltage — Main Circuit DC Voltage Too Low A.710 — Overload — High Load (Stall-level Torque) A.720 — Overload — Low Load (Continuous Over-torque) A.7A0 — Heat Sink Overheated (>100 degC)
Same family: this code belongs to the Drive & power stage alarms family - the page collects all Drive & power stage-family codes we have handled, with the common inspection order and spares.
Other brands: we also keep field-fix libraries for Universal Robots · Kawasaki · Staubli and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the YASKAWA a-400 alarm mean?

The main-circuit DC bus voltage exceeded the limit (about DC 420 V on 400 V class units); the SERVOPACK stops to protect the capacitors and power stage. In our Yaskawa service archive this alarm on DX100 / DX200 / YRC1000 systems shows up constantly, and the first on-site inspection almost always locates it. YASKAWA a-400 sits in the drive & power stage family; on our work orders the highest-frequency trigger for this code is "Incoming AC supply voltage too high".

QCan I clear a YASKAWA a-400 alarm myself?

Whether YASKAWA a-400 clears on its own depends on the outcome of "Measure incoming AC voltage and verify it is within the rated range". Finish the flow with "Replace the SERVOPACK only after supply and regeneration paths are proven healthy" to confirm YASKAWA a-400 - do not skip it. If that step is clean, the second most frequent cause on YASKAWA a-400 is "Large regenerative energy during deceleration with a weak or missing braking resistor". After clearing YASKAWA a-400, watch it for one shift. A second trip means stop and let an engineer arrive with the right test gear for the DC bus and power stage.

QFor drive/power alarms like YASKAWA a-400, what spares do you bring on site?

The commonest causes we see on YASKAWA a-400 are "Incoming AC supply voltage too high, Large regenerative energy during deceleration with a weak or missing braking resistor, Unstable plant power with voltage spikes", so we stock the van against those. For YASKAWA a-400 specifically, we stock the van along the energy chain: braking resistors, bus capacitors, input fuses, drive-board supply parts and the common power modules for the brand. For YASKAWA a-400, drive alarms usually stop the whole line, so they sit at the front of our dispatch queue. Before we roll, tell us what YASKAWA a-400 shows on the pendant alarm list (A / E / 4xxx segments) together with the servo drive's digit display and what the machine was doing - phone or video triage is free.

The notes on YASKAWA a-400 were summarized by the Henghuan Robotics repair team from on-site service. The alarm interpretation is our empirical summary for troubleshooting reference and may vary by controller and software version — please check Yaskawa documentation or contact us with questions. Henghuan is an independent service provider, not affiliated with or endorsed by Yaskawa.